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LIFTING CONTROLS CAN MEAN HIGHER PRICES, BUT FEWER SHORTAGES

JAPANESE CONTRACTORS ARE DRIVING THROUGH A DIFFICULT FRACTURED ROCK ZONE TO CONSTRUCT A SECTION OF A 12-MILE-LONG RAILROAD TUNNEL. WORKMEN ARE INSTALLING MORE THAN 200 GROUT PIPES HORIZONTALLY AROUND THE PERIPHERY OF THE TUNNEL AS A PROTECTIVE HULL TO HOLD UP THE FRACTURED ROCK. THEN THEY DRILL AND BLAST MULTIPLE SIDE DRIFTS TO EXCAVATE THE TUNNEL. ALTHOUGH THE TUNNEL IS NEARLY 90% COMPLETE, THE CONTRACTOR HAS ONLY RECENTLY STARTED MINING THE MOST DIFFICULT SECTION, THROUGH A 70-FT-LONG FRACTURED ROCK FAULT ZONE WITH ONLY 100 FT OF OVER-BURDEN. JUST BEFORE REACHING THE FAULT ZONE HE ENLARGED THE 39-FT-DIA BORE TO 50 FT. USING DRILLS SET INSIDE 100 FT-LONG GROUT PIPES, THE CONTRACTOR PLANS TO DRIVE DOUBLE ROWS AROUND THE ENTIRE PERIPHERY OF THE TUNNEL FACE. IF PROBING INDICATES THAT ROCK IS SOUND ENOUGH, HOWEVER, THE CONTRACTOR WILL DRIVE THE GROUT PIPES ONLY ALONG THE TUNNEL WALLS AND CEILING. THE 4.5-IN.-DIA PIPES ARE INSTALLED IN AN ARRAY THAT FORMS A 40-FT-DIA HULL. WORKMEN INJECT GROUT INTO THE PIPES AND BETWEEN THEM. THE CONTRACTOR IS DRIVING THROUGH THE ROCK ZONE WITH A FOUR-LEVEL SIDE DRIFT MINING METHOD FOR ADDED SAFETY. EXCAVATING AND CASTING OF THE CONCRETE OUTER LINING PROCEEDS IN STAGGERED CYCLES STARTING WITH THE BOTTOM SIDE DRIFTS. THIS METHOD IS SAFER THAN DRIVING THE 25-FT-HIGH DRIFTS IN A SINGLE ROUND OF MINING, ACCORDING TO THE CONTRACTOR. BY INSTALLING THE CONCRETE WALL LINING IN 25-FT-HIGH SECTIONS, THE TUNNEL'S CEILING SPAN IS REDUCED, AND IT IS SAFER TO MINE THE UPPER ARCH CUT IN THE NEXT STEP. THE CEILING IS BRACED BY STEEL RIB SUPPORTS ON 2-FT CENTERS OVER WHICH CREWS CAST THE 20-IN.-THICK OUTER LAYER OF LINING. THEN ABOUT THREE-PQARTERS OF THE MAIN TUNNEL FACE, EXCEPT FOR THE BOTTOM BENCH, WILL BE MINED IN A SINGLE CYCLE. FINALLY, WORKMEN WILL DRILL AND BLAST THE REMAINING BOTTOM SECTION AND INSTALL A 40-IN.-THICK CONCRETE INVERT AND CAST THE 20-IN.-THICK INNER LINING. IF ROCK CONDITIONS ARE EXCEPTIONALLY BAD THE CONTRACTOR WILL HAVE TO USE A MORE ELABORATE THREE-DRIFT METHOD AFTER EXCAVATING THE UPPER RING. IN THIS CASE WORKMEN WOULD MINE A 10 X 10.5-FT CENTER DRIFT AT THE BOTTOM FIRST AND CAST THE INVERT FOR THAT SECTION. THEN TWO 7.5 - FT-WIDE SIDE DRIFTS WILL BE MINED IN A PARALLEL OPERATION AND THE INVERT CAST AS THE EXCAVATION PROCEEDS. THE TUNNEL FACE WOULD BE MINED IN A SINGLE CYCLE FOLLOWED BY CASTING THE INNER CONCRETE LINING ON THE WALLS AND CEILING.